A typical spray booth line for painting car bodies is divided into a plurality of successive sections which by means of fans are supplied with vertical supply air flows through filter roofs provided in the sections. In those sections where the car bodies are sprayed with paint, exhaust air flows polluted with paint particles and solvents are evacuated by means of fans. In order that polluted air should not spread horizontally from the spray sections to the other sections, it is vital that the horizontal air flows between the sections be directed towards the spray sections. It is also vital that the horizontal air flows be small in relation to the supply and exhaust air flows so as not to create secondary eddies in the spray sections or entail that paint particles are transferred from one car body to another. The horizontal air flows are suitably regulated by such a speed control of the supply air fans that the size and the direction of the horizontal air flows will become as desired. To permit carrying out this control in optimum fashion, the horizontal air flows must be accurately measured.
Thermal flow transducers, i.e. transducers having a measuring body which should be placed in the flow to be measured, and a means for heating the measuring body, and relying in different ways on the fact that the flow cools the heated measuring body, are known in various designs. However, none of the prior-art flow transducers can be considered to comply with the requirements 1-3 which are listed below and which must be placed on a flow transducer to be used in a spray booth line of the type described above.
1) Since a spray booth line is classified as explosive environment, the measuring body of the flow transducer may only be heated to a temperature which but insignificantly exceeds the ambient temperature. PA1 2) The flow transducer must be relatively insensitive to deposits of paint and other chemicals on the measuring body. PA1 3) The flow transducer must have a high sensitivity in the range around zero, especially in the range -2 m/s-+2 m/s.